So far, we've been focused on the use of HD video in ePub documents but what about SD? Standard definition video is still quite common, especially for older works produced for television. Whereas HD has an aspect ratio of 1.78:1 or 16:9, SD has an aspect ratio of 1.33:1 or 4:3.
As we have seen, iOS devices (iPad, iPhone and iPod touch), introduce black bars (letterbox, pillarbox and windowbox) and cropping/scaling in order to maintain the aspect ratio of the source video. Without these, the video we see on these devices would be stretched or squished which is not what the author intended. This is due to the fact that iOS devices do not have 16:9 screens. The iPad is 1024x768 which is 1:33:1 or 4:3. The iPhone and iPod touch are 480x320 (older) or 960x640 (newer, retina displays) which is 1.5:1 or 3:2. In other words, iOS devices have screens with aspect ratios that are the same as or very close to standard definition TV dimensions.
I've created the following screencast to illustrate the many ways that the iBooks application on these iOS devices will present Standard Definition (SD) video. There's a lot to account for with different orientations (portrait vs landscape) and whether the reader chooses to view video embedded in an EPUB-based eBook in situ, proportional full screen or non-proportional full screen. It's best to see all of this in action.
If your source is SD, QuickTime X Player will maintain that aspect ratio in all of its export options. This is ideal for video embedded in EPUB-based eBooks. Playback at 640x480 minimizes the use of black bars, cropping and scaling, the iPod bit rate keeps a good balance between file size and fidelity in most cases and the MPEG-4 Baseline Profile assures that the video will play wherever the iBooks.app is found.